Typical hard disk drives include one or more vertically-aligned rotating information storage disks, each having a pair of associated magnetic read/write transducers, or heads, adapted to transfer information between the disk and an external computer system. One of the heads communicates with the upper surface of the disk, while the other head communicates with the lower surface of the disk. The information storage disks are journaled about a spindle motor assembly capable of rotating the disks at high speeds. In conventional disk drives, the read/write heads are carried by a plurality of vertically-aligned resilient metal blades, each commonly known as a head suspension arm. Each head is attached by a gimbal mechanism to a slider, which is in turn secured, e.g. by epoxy, to a planar surface of the head suspension arm's end. The plurality of suspension arms and heads are carried by an E-block actuator, which is in turn coupled to a precision assembly of bearings and a shaft. Typically, the E-block is constructed of die cast aluminum alloy. The combination of read/write heads, head suspension arms, and E-block actuator is commonly known as a head stack assembly (HSA), and is illustrated in FIG. 1b.
In the typical operation of such disk drives, the head does not actually ride on the surface of the rotating disk but instead is separated by an extremely thin layer of air. As the disk rotates at high speeds (e.g. 5400 rpm), a thin air bearing (e.g. 2-4 micro inch) forms between the rotating disk and the slider. The slider is aerodynamically shaped so that the head effectively rides, or flies, on the air bearing, which tends to lift slider upward and away from the disk surface. To maintain a fixed distance between the head and disk, commonly referred to as "fly height", a continuous biasing force, commonly referred to as a "gram load" is applied to the head suspension arm urging the head toward the disk surface, thereby enabling the head to counteract the upward force of the air bearing. Typically, the gram load is approximately 3-6 grams, (as measured when the heads are contacting the disk), depending on the flying characteristics of the slider. The gram load is provided by incorporating a preloaded spring portion, e.g. a bend, to the suspension arm to bias the head toward the disk.
With the ever increasing demand for higher data storage capacity in disk drives, it has become even more essential to maintain the head at a constant low fly height above the disk. Hence, it is critical that the head stack assembly is manufactured with a proper gram load.
In the prior art, known systems for measuring gram load of the suspension arm were time and labor intensive. FIG. 1a shows a typical gram load measuring system 100. In the prior art, the system 100 included a computer 110 for storing gram load measurements, a gram load measuring instrument 130, and an apparatus 120 for manually placing each of the suspension arms of a head stack assembly into position to measure the gram load. FIG. 1c illustrates the apparatus 120, including a pin tower assembly 60 comprising a plurality of vertically aligned cam pins 62 for meshably engaging suspension arms 75 such as those of the head stack assembly 70 shown in FIG. 1b. As the pins and the suspension arms engage, shown in FIG. 1b, each cam pin 62 is situated between a top and bottom arm. Each cam pin 62 includes an L-shaped lever arm 64 integrally press fit at one end. Referring back to FIG. 1a, the incrementally varying lengths of the pins 62 form a step-like staggered array of levers, enabling each to be rotated without interference from adjacent levers. To measure the gram load of each arm, each lever arm had to be manually rotated. For example, each lever was first manually rotated 90 degrees in the clockwise direction, placing the top suspension arm in contact with a substrate (e.g. glass substrate), which prevented adjacent heads from contacting each other. The operator would then manually prompt the measuring apparatus 130, to measure the gram load in the suspension arm. Typically, the measuring apparatus included a load cell which was mounted to the substrate. Upon completion of measuring the gram load, the operator would then be required to manually prompt the computer 110 to store the gram load measurement. Then, the pin lever 64 is again manually rotated 180 degrees in the other direction, returning the top suspension arm to its original position, and placing the bottom suspension arm in contact with the adjacent substrate. Again the operator is required to manually prompt the measuring apparatus 130 to measure the gram load in the suspension arm, then prompt the computer 110 to store that measurement. This process of manually rotating the lever arm, then prompting the measuring apparatus 130 and the computer 110 is repeated until the gram load in every suspension arm is measured. Clearly the disadvantage of this prior art system is the time and labor consuming nature of the apparatus 120. In addition, the possibility of operator error adds to the drawbacks of the prior art system.
Thus, a hitherto unsolved need has remained for an automated system for measuring the gram load of a suspension arm in a head stack assembly which reduces operator dependency.